On the Size of a Ricci Flow Neckpinch via Optimal Transport

In this paper we apply techniques from optimal transport to study the neckpinch examples of Angenent-Knopf which arise through the Ricci flow on ${\mathbb{S}}^{n+1}$. In particular, we recover their proof of ‘single-point pinching’ along the flow. Using the methods of optimal transportation, we are able to remove the assumption of reflection symmetry for the metric. Our argument relies on the heuristic for weak Ricci flow proposed by McCann-Topping which characterizes super solutions of the Ricci flow by the contractivity of diffusions.